1. Field of the Invention
This invention relates to an improved method for controlling a catalytic hydrocarbon conversion process. More particularly, this invention relates to a method for controlling the catalyst circulation rate in a fluid catalytic cracking process by employing a low pressure drop valve means in the spent catalyst circuit in conjunction with a control riser.
2. Description of the Prior Art
The fluidized catalytic cracking of hydrocarbons in well known in the prior art and may be accomplished in a variety of processes which employ fluidized solid techniques. Normally in such processes, suitably preheated, relatively high molecular weight hydrocarbon liquids and/or vapors are contacted with hot, finely divided, solid catalyst particles either in a fluidized bed reactor or in an elongated riser reactor and maintained at an elevated temperature in a fluidized state for a period of time sufficient to effect the desired degree of cracking to the lower molecular weight hydrocarbons typical of those present in motor gasolines and distillate fuels.
During the cracking reaction, coke is deposited on the catalyst particles in the cracking zone thereby reducing the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stock. In order to restore a major portion of activity to the coke contaminated or spent catalyst, the catalyst is transferred from a cracking zone into a regeneration zone wherein the catalyst is contacted with an oxygen-containing regeneration gas, such as air, under conditions sufficient to burn at least a portion, preferably a substantial portion, of the coke from the catalyst. The regenerated catalyst is subsequently withdrawn fron the regeneration zone and reintroduced into the cracking zone for reaction with additional hydrocarbon feedstock.
In commercial catalytic cracking units, good control of the circulating catalyst stream is essential for economic operation. Erratic flow causes fluctuations in the quality of the regenerated catalyst, the catalyst/oil ratio, the catalyst inventory in the cracking zone and the temperature of the cracking zone -- all of which have important effects on the product yields and on the feed capacity of the unit. In general, the catalyst circulation rate in present catalytic cracking units is controlled by using slide valves or other similar devices in the catalyst circulation lines (see, for example, U.S. Pat. Nos. 2,593,339; 3,001,931; 3,033,780; 3,769,203; 3,847,793) or by using a dynamic pressure balance control system such as is described in U.S. Pat. No. 2,589,124. The present invention is primarily concerned with an improvement in the method described in U.S. Pat. No. 2,589,124, the disclosure of which is incorporated herein by reference.
According to U.S. Pat. No. 2,589,124, cracking catalyst is circulated between the cracking zone and the regeneration zone without the use of control slide valves. Rather, the rate of flow of solids between the two zones is regulated by controlling the density of the catalyst suspension in a control riser leading to the regeneration zone. The change in density may be effected by injecting variable amounts of a control gas, such as air, into the control riser. Since the pressures at the top and bottom of the control riser correspond substantially to the pressures of the regeneration zone and cracking zone, respectively, the change of density within the riser must be compensated for by a change in the mass velocity of the catalyst stream flowing therein. However, control gas velocities in the riser which permit stable control of the catalyst circulation rate therein are limited to a fairly narrow range, i.e., from between about 6 to about 15 ft./second. For example, at control gas velocities of less than about 6 ft./second, large bubbles, i.e., slugs of gas, form in the riser which adversely affect the stability of catalyst flow. At control gas velocities of greater than about 15 ft./second, the frictional pressure drop of the catalyst stream flowing therein increases with increased gas velocity as fast as the pressure drop resulting from the lower static head due to the dilution of the catalyst by additional gas is reduced. Thus, there is relatively little change in the catalyst circulation rate. In fact, the catalyst circulation can become "choked" resulting in a lower catalyst circulation rate at increased gas velocities.
In either of the methods for controlling catalyst circulation rate mentioned above, the circulation rate can also be varied by shifting the pressure balance between the regeneration zone and the cracking zone, i.e., the regeneration zone pressure can be raised or the cracking zone pressure can be reduced, while maintaining the flow of gas into the control riser in a range which provides stable catalyst flow. However, an increase in regeneration zone pressure will require an increase in utility consumption since the blower supplying the regeneration gas to the regeneration zone must compensate for the increased pressure. Similarly, if the pressure of the cracking zone were reduced, the requirements of the compressor associated with light ends recovery of the vapor stream from the fractionation zone distillate drum will be increased.
Thus, it would be desirable to have available a simple, economical method to control the catalyst circulation rate of a fluid catalytic cracking process wherein the disadvantages presently associated with the prior art are eliminated.